New Mexico Mineral Symposium — Abstracts

Carbonatites are unique carbonate-rich rocks of apparent magmatic origin that are characterized by a distinct but variable mineralogy, composition, and association alteration. In the Lemitar Mountains, central New Mexico, Paleozoic carbonatite dikes (minimum age 449±16 m.y.) intrude Precambrian granites, diorite/gabbro, metamorphic rocks, and amphibolite dikes. They contain greater than 50% carbonate minerals (calcite, dolomite, and ankerite) and varying amounts of apatite, magnetite, biotite, and other accessory minerals.

Cathodoluminescence (CL) is the characteristic visible radiation (color) produced in a mineral subjected to a bombardment of electrons. Many features of a sample observed under CL are not seen using either standard optical petrographic techniques or using an electron microprobe; this is especially true of carbonatites. Luminescence as observed under a CL stage is manifested as different colors and intensities than are seen under the electron microprobe. This is a consequence of the different current densities of the electron bombardment, about 0.1 to 1 A/m² for typical CII-stage operation compared to 102 A/m² (10 pm diameter beam) to 104 A/m² (1 μm beam) on the microprobe. Thus quartz, which exhibits no CL, appears orange pink under the microprobe whereas normally red CL calcite appears to show only weak luminescence under the microprobe.

The Lemitar carbonatites exhibit bright-red CL, characteristic of carbonatites elsewhere in the world. The bright-red luminescence is due to compositional variations in fine-grained carbonate minerals. For example, calcites that luminesce red contain variable FeO and 1-2% MnO; non-luminescing dolomites contain variable FeO and 0.03-0.8% MnO. Color zonation in vein calcite under CL is related to variation in MnO from no detectable MnO in lighter orange CL calcite to 0.5% MnO in bright-red CL zones. Apatites luminesce blue to green-gray to gray in carbonatites and are typically zoned whereas apatites in unaltered country rock luminesce yellow. Electron microprobe studies reveal that bright-blue apatites are enriched in SrO (about 1.8% SrO) relative to gray luminescing apatites (about 0.4% SrO) and yellow luminescing apatites (no detectable SrO). Other elements (such as rare-earth elements) in addition to Sr may cause the difference in luminescence color of apatite when exposed to CL.

Cathodoluminescence and electron microprobe studies of opaque grains reveal complex intergrowths of magnetite, ilmenite, rutile, leucoxene, calcite, and quartz; zoning within the magnetite is evidenced by red CL of the inclusions. Additional minerals detected by the electron microprobe include Nb-bearing rutile, Nb-bearing titanite, chalcopyrite, and pyrite or pyrrhotite. Cathodoluminescence reveals zoning of fluorite that is not seen with the normal petrographic microscope.